1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexing) transmission system for transmitting signals of a plurality of channels.
2. Related Background Art
WDM transmission systems enables high-speed and large-capacity optical transmission over the long haul by transmitting multiplexed signals (hereinafter referred to as WDM signals) of a plurality of channels. For high-speed and large-capacity optical transmission over the long haul in the WDM transmission systems, it is important to suppress deterioration of signal waveform due to the nonlinear optical phenomena in the optical transmission lines and it is also important to suppress deterioration of signal waveform due to accumulated chromatic dispersion of the optical transmission lines. A number of research and development has been conducted heretofore from these viewpoints.
For example, xe2x80x9cS. Tsuda, et al., xe2x80x9cTransmission of 80xc3x9710 Gbit/s WDM channels with 50 GHz spacing over 500 km of LEAF(trademark) fiber,xe2x80x9d OFC2000, TuJ6 (2000)xe2x80x9d (Document 1) reports the WDM transmission experiment carried out using nonzero-dispersion shifted optical fibers (NZ-DSFs) with chromatic dispersion of 4 ps/nm/km at the wavelength 1550 nm. xe2x80x9cS. Bigo, et al., xe2x80x9c1.5 Terabit/s WDM transmission of 150 channels at 10 Gbit/s over 4xc3x97100 km of TeraLight(trademark) fiber,xe2x80x9d ECOC""99, PD2-9 (1999)xe2x80x9d (Document 2) reports the WDM transmission experiment carried out using nonzero-dispersion shifted optical fibers with chromatic dispersion of 8 ps/nm/km at the wavelength 1550 nm.
xe2x80x9cC. Furst, et al., xe2x80x9cPerformance limits of nonlinear RZ and NRZ coded transmission at 10 and 40 Gb/s on different fibers,xe2x80x9d OFC2000, WM31 (2000)xe2x80x9d (Document 3) reports the computation result that at the bit rate of 10 Gbit/s or 40 Gbit/s, the nonzero-dispersion shifted optical fiber with the absolute value of chromatic dispersion of about several ps/nm/km at the wavelength 1.55 xcexcm has a larger signal power margin than standard single-mode optical fibers having the zero-dispersion wavelength near the wavelength of 1.3 xcexcm. This Document 3 also describes that the deterioration of signal waveform due to interaction between chromatic dispersion and self-phase modulation (SPM) being one of the nonlinear optical phenomena becomes prominent with increase in the absolute value of chromatic dispersion of the optical transmission line.
The inventors have studied the above prior arts in detail and, and as a result, have found problems as follows.
Namely, the above Documents 1 to 3 have reported the results of research for improvement in transmission performance of the WDM transmission systems. However, none clarified the optimum conditions of the WDM transmission systems for excellent transmission characteristics yet.
For example, when the absolute value of chromatic dispersion of the optical transmission lines is small (for example, about 4 ps/nm/km) as in the case wherein the nonzero-dispersion shifted optical fibers are used as optical transmission lines, since the quasi phase matching condition will be satisfied among the WDM signals if further increase of capacity is achieved by narrowing the channel spacing between the WDM signals (wavelength spacing between signal channels), there easily occurs the deterioration of signal waveform due to four-wave mixing (FWM) being one of the nonlinear optical phenomena. In addition, the signals also becomes more susceptible to cross-phase modulation (XPM), so that the waveform deterioration becomes much easier to occur.
On the other hand, when the absolute value of chromatic dispersion of the optical transmission lines is large (for example, about 20 ps/nm/km) as in the case wherein the standard single-mode optical fibers are used as optical transmission lines, since the WDM signals each keeping peak power high in a relatively long distance because of pulse compression propagate over the long haul in the optical transmission lines, there sometimes appears considerable deterioration of signal waveform due to not only the effect of FWM but also the interaction between SPM or XPM and chromatic dispersion. Since the absolute value of accumulated chromatic dispersion of the optical transmission lines becomes very large, a necessary dispersion compensation amount becomes larger, so as to raise difficulties in system design and increase the cost.
The present invention has been accomplished in order to solve the above problems and an object of the invention is to provide a WDM transmission system having a structure enabling high-speed and large-capacity optical transmission over the long haul.
A WDM transmission system according to the present invention includes an optical transmission line for transmitting WDM signals in four or more channels with a channel spacing therebetween of not more than 0.4 nm and a bit rate of not less than 9 Gbit/s. This optical transmission line is disposed between a transmitter and a receiver and, in addition, can be disposed at least one of between a transmitter and a repeater, between repeaters, and between a repeater and a receiver if one or more repeaters (including an optical amplifier, or the like) are disposed between the transmitter and the receiver.
Particularly, the optical transmission line has, as characteristics at a wavelength of 1.55 xcexcm, a chromatic dispersion with an absolute value of 8-16 ps/nm/km and a ratio (n2/Aeff), which is a ratio of a nonlinear refractive index n2 to an effective area Aeff of the optical transmission line, of not more than 6.4xc3x9710xe2x88x9210/W. The optical transmission line preferably has a transmission loss of not more than 0.3 dB/km. By introduction of this optical transmission line to the WDM transmission system, a requirement level for system design can be decreased and a low-cost system having a stable transmission quantity construction can be easily constructed.
Accordingly, the WDM transmission system suppresses the deterioration of signal waveform due to the FWM, which can occur when the absolute value of chromatic dispersion of the optical transmission line is small, and suppresses the deterioration of signal waveform due to the interaction between chromatic dispersion and SPM or XPM, which can mainly occur when the absolute value of chromatic dispersion of the optical transmission line is large, so as to implement the high-speed and large-capacity optical transmission over the long haul.
In the WDM transmission system according to the present invention, the optical transmission line preferably has the chromatic dispersion with the absolute value of 9-14 ps/nm/km and more preferably 10-12 ps/nm/km. In these cases, particularly, the deterioration of signal waveform due to the FWM is sufficiently suppressed, and the deterioration of signal waveform due to the interaction between SPM or XPM and chromatic dispersion is sufficiently suppressed.
In the WDM transmission system according to the present invention, spectral efficiency is preferably not less than 0.1 bit/s/Hz and more preferably not less than 0.2 bit/s/Hz. In these cases the system enables large-capacity optical transmission and even in these cases, the deterioration of signal waveform can also be effectively suppressed when the WDM transmission system meets the aforementioned requirements. Here the spectral efficiency is defined as a ratio (bit rate/channel spacing) of the bit rate (unit: bit/s) to the channel spacing (unit: Hz) between the multiple channels for the WDM signals.
In the WDM transmission system according to the present invention, each repeater section by defining a setting interval between optical amplifiers, namely the optical transmission line disposed between a plurality of optical amplifiers during these optical amplifiers are disposed in the WDM transmission system, is preferably not less than 20 km and more preferably not less than 40 km. In these cases the system enables long-haul optical transmission and even in these cases, the deterioration of signal waveform can also be effectively suppressed.
The WDM transmission system according to the present invention can further comprises a dispersion compensator for compensating for chromatic dispersion of the optical transmission line at the wavelengths of the WDM signals. In this case, since the chromatic dispersion of the optical transmission line is compensated for by the dispersion compensator, the absolute value of accumulated chromatic dispersion of the whole including the optical transmission line and the dispersion compensator becomes small, so as to further suppress the deterioration of signal waveform. Preferably, when the dispersion compensator is provided every repeater section, adding or dropping of a part of signals can be easily performed in a repeater.
It is also preferable that a dispersion slope compensation ratio of the dispersion compensator is approximately 100%. Here the dispersion slope compensation ratio (unit: %) is defined as 100xc3x97(S2/D2)/(S1/D1), where D1 is the chromatic dispersion of the optical transmission line, S1 the dispersion slope of the optical transmission line, D2 the chromatic dispersion of the dispersion compensator, and S2 the dispersion slope of the dispersion compensator, at each wavelength of the WDM signals. In this case, the chromatic dispersion of the optical transmission line is compensated for by the dispersion compensator while the dispersion slope of the optical transmission line is also compensated for by the dispersion compensator. Therefore, a variation of each waveform deterioration of the WDM signals can be decreased and this enables signal transmission in a broad band.
In the WDM transmission system according to the present invention, the optical transmission line preferably has a polarization mode dispersion of not more than 0.2 psxc2x7kmxe2x88x92xc2xd. In this case, the deterioration of signal waveform due to the polarization mode dispersion is suppressed, which is suitable for high-speed and large-capacity optical transmission over the long haul. When the WDM transmission system is provided with an optical amplifier, optical amplification can be implemented with stable gain for each of the WDM signals even if the gain of optical amplification for the WDM signals in this optical amplifier has polarization dependency.
In the WDM transmission system according to the present invention, each power of the WDM signals entering the optical transmission line is not less than 1 dBm. In this case, it is feasible to enhance the stability of the whole system because of expansion of the signal power margin and it is also feasible to decrease the number of optical amplifiers installed and thus decrease the cost because of increase of loss budget in transmission.
Further, at each wavelength of the WDM signals (in the multiple channels with channel spacing therebetween of not more than 0.4 nm and bit rate of not less than 9 Gbit/s), the optical transmission line preferably has a chromatic dispersion with an absolute value of 8-16 ps/nm/km. In this case, the waveform deterioration is suppressed throughout the entire signal wavelength band and thus the number of channel can be increased or decreased without cumbersome redesigning of the system.
In the WDM transmission system according to the present invention, the optical transmission line preferably has, at the wavelength of 1.58 xcexcm, the chromatic dispersion with the absolute value of 8-16 ps/nm/km. In this case, not only in the 1.55 xcexcm wavelength band but also in the 1.58 xcexcm wavelength band, the system suppresses the deterioration of signal waveform due to the FWM, which can occur when the absolute value of chromatic dispersion of the optical transmission line is small, and also suppresses the deterioration of signal waveform due to the interaction between chromatic dispersion and SPM or XPM, which can occur when the absolute value of chromatic dispersion of the optical transmission line is large. As a result, this enables the higher-speed and larger-capacity optical transmission over the long haul.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.